1. Field of the Invention
The present invention relates to a catalytic method for purifying exhaust gases of an internal combustion engine. It particularly relates to such a catalyst employing a zeolite.
2. Description of the Related Art
A zeolite has pores whose sizes are almost equal to those of molecules, and is also known as a "molecular sieve". The zeolite has been utilized in many reactions as a catalyst as well as an absorbent. Further, the zeolite is also utilized as a cation exchanger, since the zeolite includes cations for neutralizing negative electric charges of Al.sub.2 O.sub.3, and since the cations are easily exchanged with the other cations in an aqueous solution.
In recent years, it has been examined to apply the zeolite to a catalyst for purifying exhaust gases of automobiles in view of these characteristics of the zeolite.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 97047/1985 discloses a catalyst for purifying exhaust gases in which copper is loaded into a zeolite by means of an ion exchange.
Further, Japanese Unexamined Patent Publication (KOKAI) No. 135541/1989 discloses a catalyst for purifying exhaust gases in which noble metals selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir) and ruthenium (Ru) are loaded into a zeolite by means of an ion exchange. In the catalyst for purifying exhaust gases, a catalyst support is coated with 100 g of the zeolite per 1 liter of the catalyst support, and 1.0 g of platinum or palladium and 0.2 g of rhodium per 1 liter of the catalyst support are loaded on the zeolite, as usual. Since the noble metals are expensive, the amounts thereof used as a catalyst metal are usually not so great.
These catalysts offer an improved conversion performance on nitrogen oxides even under a lean atmosphere where oxygen exists in an excess amount. Accordingly, it is possible to dilute the concentration of the air-fuel mixture supplied into an engine, thereby enabling to sufficiently purify harmful components over a wide range from the theoretical air-fuel ratio to the lean atmosphere side. Therefore, it is possible to achieve a higher mileage with a less fuel consumption.
However, in the above-mentioned catalyst in which the zeolite contains copper as positive ions, it has been found that the copper is aggregated by the heat generated during the service as a catalyst, and that the catalyst performance deteriorates because the copper is unstable at a high temperature of 800.degree. C. or more.
Further, in the above-mentioned catalyst in which the zeolite contains the noble metals as cation, it has been found that the catalyst performance deteriorates after a durability test.